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Molybdenum heteropoly acid chemistry with its inherent amplification factor has been used to improve the sensitivities and detection limits of methods for phosphorus and germanium determination, using both flame and electrothermal atomization atomic absorption spectrometry to determine the molybdenum. Phosphorus extracted as 12-molybdophosphoric acid, was used as a model element for the study of certain aspects of the procedure. Solvent extraction was used for the separation of the heteropoly species from the considerable excess of molybdate (added to drive the reactions to completion), which usually produces high blank values in both flame and electrothermal atomization work. A considerable improvement in blank levels, to below the instrumental detection limits has been achieved by careful drying of the extract. In addition to the chemistry of formation and extraction, the flame atomization of molybdenum has been investigated with view of improving sensitivity and detection limit. Modifications to the flame gas composition by the addition e.g. possible additional reducing agents, have been investigated. Variation of the particle microenvironment in the flame by varying the salt composition of the solution was also studied. Improvement in the nebulization process by combining flow injection sample introduction with modification of the solution physical properties was investigated.

The work presented in this thesis describes the results of Molecular Dynamics (MD) simulations applied to the interaction of silver clusters with graphite surfaces and some numerical and theoretical methods concerning the extension of MD simulations to longer time scales (hyper-MD). The first part of this thesis studies the implantation of clusters at normal incidence onto a graphite surface in order to determine the scaling of the penetration depth (PD) against the impact energy. A comparison with experimental results is made with good agreement. The main physical observations of the impact process are described and analysed. It is shown that there is a threshold impact velocity above which the linear dependence on PD on impact energy changes to a linear dependence on velocity. Implantation of silver clusters at oblique incidence is also considered. The second part of this work analyses the validity and feasibility of the three minimisation methods for the hyper-MD simulation method whereby time scales of an MD simulation can be extended. A correct mathematical basis for the iterative method is derived. It is found that one of the iterative methods, upon which hyper- NID is based, is very likely to fail in high-dimensional situations because it requires a too expensive convergence. Two new approximations to the hyper-MD approach are proposed, which reduce the computational effort considerably. Both approaches, although not exact, can help to search for some of the most likely transitions in the system. Some examples are given to illustrate this.

Cross sections for electron capture by (^4)He(^2+) ions from ground state atomic hydrogen are presented for a (^4)He(^2+) laboratory energy range from 1 to 800 keV (0.25 to 200 keV amu (^-1)). The cross sections were calculated using a coupled channel approximation in which the electronic wavefunction was expanded in terms of a finite number of atomic orbital basis states centred upon the target and the projectile. Electron translation factors which incorporated a switching function were included in the basis states. The semi-classical impact parameter approximation was employed. The cross sections presented are for electron capture into the 2s state of (^4)Me(^+), and into the n = 2 level of (^4)He(^+) using two states and four states respectively in the basis expansion. Four functional forms of switching function were used in the translation factors. The cross sections are compared with ones calculated using two-state and four-state atomic basis expansions which used plane-wave translation factors, and also with other theoretical and experimental cross sections. For energies ≤ 2.5 keV amu (^-1) fairly reasonable agreement is obtained with other data. For energies ≥ 2.5 keV amu (^-1) the present cross sections are in poor to extremely poor agreement with other data, steady divergence of the present results from existing data being observed with increasing energy. The present results are discussed, and conclusions and suggestions for future work are made.

The results of close coupling (CC) and infinite order sudden (IOS) approximation calculations of cross sections for rovibrational excitation of both para and ortho H(_2) by He are presented. Large discrepancies are found between the present CC results and those of Lin and Secrest (1979) and Lin (1979). The v = O → 1 vibrationally inelastic cross sections are found to differ from those of Lin by factors attaining four orders of magnitude close to the v = 1 excitation threshold. Also, structure in the variation of both vibrationally elastic and inelastic cross sections with energy, reported by Lin and Secrest, and Lin, is absent in the present results. The present CC results are found to be in good quantitative agreement with the coupled states calculations of Alexander and McGuire (1976). Agreement with the IOS calculations is only qualitative but improves with increasing collision energy, consistent with the progressive failure of the energy sudden component of the IOS approximation as the collision energy falls. The values of the vibrational relaxation rate coefficient calculated from the CC results fall below the experimental data of Audibert et al. (1976) at low temperature. This is most probably due to the relatively poor description of the H(_2) system employed, in particular the interaction potential of Gordon and Secrest (1970). The CC results are employed to investigate the accuracy of two energy sudden factorisation schemes. The factorisation which includes off-energy-shell effects is shown to be more accurate than that which does not. However, neither scheme produces cross sections which obey detailed balance. The present IOS results are in good agreement with the adiabatic distorted wave IOS calculations of Bieniek (1980) at low energy. However, as the collision energy increases significant discrepancies appear. For H(_2) + He it appears that at energies sufficiently high for the IOS approximation to be valid the use of adiabatic distorted wave techniques is not valid. Exploratory IOS calculations of rovibrational excitation of H(_2) by h(^+) are reported and discussed. There appears to be evidence that the comparison between theoretical and experimental values of rovibrational cross sections presented by Schinke et al. (1980) and Schinke (1980) is distorted by their restricted numerical methods and faults in their basis wavefunctions.

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